Various types of fuel cells can be distinguished on the basis of the type of electrolyte employed. For the mobile field of application, i.e. for the generation of electric power for vehicles driven by electric motors, fuel cells with proton-conducting membranes have proven effective, whose operating temperature typically lies in the range of 60-80.degree. C. To be sure membranes with fuel cells have already been devised which operate stably both at room temperature (EP 0 827 228 A1) and also at temperatures between 100 and 200.degree. C. Such a membrane is described in U.S. Pat. No. 5,716,727.
Presently vehicles using fuel cells have fuel cell systems with a power of about 20 to 50 kW. The decisive handicap is presently the fact that too long a time passes after the vehicle starts operating until a stack reaches its operating temperature where it can produce full power. This problem becomes greater when fuel cells with higher operating temperatures are used. In other words, it is expected that the time delay for these fuel cells would be even greater.
Another problem to be considered is the fact that, for example, in the operation of oxide ceramic fuel cells (SOFC: Solid Oxide Fuel Cells), as is described in DE 196 11 591 A1, the temperature spread, i.e. the difference between the gas exit temperature and the gas intake temperature must not become too great since otherwise the material of the fuel cell would be very strongly loaded and inhomogeneous reaction distributions occur. It is therefore necessary to bring the process gas as well as the cooling gases or the cooling air up to approximately operating temperature before they enter the stack. DE 196 11 591 A1 therefore proposes that two oxide ceramic fuel cells of somewhat different types (one having a metallic and the other a ceramic bipolar plate) be connected in series, such that the process gases and the cooling air first enter the fuel cell with the lower (800.degree. C.) and then the fuel cell with the higher operating temperature (1000.degree. C.)